In November, while updating the House of Common on the security outlook following the terrorist attacks in London and Glasgow a few days after he became Prime Minister last July, Mr Brown told MPs that the group were in the process of being deported.

Eight months on, none of the 24 has left the country – and the majority are not even in detention.

The admission that all of the suspects referred to by the Prime Minister in his statement to MPs have yet to be deported was slipped out by the Home Office during Parliament’s summer recess, when most MPs are on holiday.

It came in response to a written question posed by James Clappison, a Conservative MP, two months ago. Written Parliamentary questions from MPs are usually answered by ministers within a few days.

In his response, Liam Byrne, the Home Office Minister, said that 12 of the suspects, all Libyan nations, could not be deported after two of them, known only as “DD” and “AS” brought a successful challenge at the Court of Appeal.

He added: “Of these 12, eight previously had the right to remain in the UK and continue to do so.

“All 12 of the cases are under review as to their future immigration status. Twelve others remain subject to ongoing deportation proceedings and are at various stages of the appeals process.”

One of the Libyans is the brother-in-law of the ringleader of the Madrid bombers responsible for 191 deaths in 2004, and was described in court as a “global jihadist with links to al-Qa’eda.”

Another is an Islamist extremist said to be “clearly engaged in support of jihadist activities”.

A number of Algerians in the group of 24 have also recently won the right to appeal against deportation. Their case will be heard in the House of Lords.

With the exception of Abu Qatada, the Palestinian-Jordanian “preacher of hate,” all of the group have been granted guaranteed anonymity.

Once described as Osama bin Laden’s main operative in Europe, Qatada was released from jail in June after the Government failed to deport him to Jordan, where he is wanted on terrorist offences, and confined to his house while awaiting the results of a House of Lords appeal.

He has been photographed strolling around his neighbourhood during the two hours a day he is allowed out under the terms of his bail conditions.

The Qatada family claim an estimated Â£47,000-a-year in benefits and have been provided with a council house worth around Â£800,000.

The ruling last April that the Libyans could not be deported undermined Mr Brown’s promise to MPs that the Government would obtain agreements with other countries to accept foreign-born terror suspects.

At the time, David Cameron, the Conservative leader, warned the Prime Minister that the deportations might never happen if the suspects brought successful cases under the Human Rights Act, but his concerns were brushed aside.

Mr Brown said: “I want to update the House, as I promised in July, on the measures we are taking at home to root out terrorism and strengthen the resilience of communities to resist extremist influences following the incidents of 29 June and 30 June.

“In addition to the nine foreign nationals recently deported under immigration powers on grounds of national security, a further 24 foreign nationals are currently subject to deportation proceedings on national security grounds.”

Mr Cameron responded: “The Prime Minister spoke about the need to deport those who put our national security at risk. Does he agree that we have got to will the means as well as willing the end? If he is advised that the Human Rights Act gets in the way of deportation, will he agree to its replacement?”

Mr Brown did not answer the question, and, despite Mr Cameron’s warning, the Government has been unable to find a solution to the legal problem of deporting the suspects.

Most if not all of the group are currently on immigration bail, with at least 12 under night-time curfew.

Dominic Grieve, the shadow home secretary, said: “David Cameron challenged Gordon Brown about this last November.

“Yet again the Prime Minister made a grand statement but proved himself unable to deliver, further putting the security of this nation at risk.”

Mr Clappison added: “This is yet another failure by the Government to keep its word on deporting terrorist suspects.

“It is also another massive strain on our security services to monitor these dangerous individuals.”

A Home Office spokesman said: “It is the Government’s position that individuals who are not British nationals and who pose a threat to the national security of the United Kingdom should be deported”.